Separation of hydrocarbons



July l2, 1949. A. E. ROBERTSON SEPARATION OF HYDROCARBONS Filed Aug. 4,1945 .uzzwnuo 10.1 I

056.... mrinnmo I GbborQel-I Patented July 12, 1949 SEPARATION FHYDROCARBONS Anthony E. Robertson, Roselle, N. J., asslgnor to StandardOil Develo poration of Delaware pment Company, a cor- Application August4, 1945, Serial No. 608,975

This invention relates to the separation of highly branched hydrocarbonsfrom the less highly branched, and particularly, to the separation ofthe highly branched hydrocarbons from the less highly branchedhydrocarbons by diffusion.

It is known that when a mixture of two dissimilar liquids is allowed tocontact a membrane the membrane normally hinders the passage of one ofthese liquids to a greater extent than the other and pressure differencemay be established between the two sides. Mineral oils have been freedof paramns, polymerized products and Water by dialyzing the oil in asolvent with a membrane which is made by dipping a support (of cloth)into a bath of rubber.

It is also known that hydrocarbons may be separated by diffusion ordialysis but these separations heretofore have been based on either (a)differences in speed of diffusion as caused by differences in molecularweight, or (b) differences in the solubility of the materials beingseparated in the diaphragm. Prior art has shown the feasibility ofseparating hydrocarbons having different solubility characteristics frommixtures, for example, butene-2 from butene-butane mixture, by allowingthe hydrocarbons in the gaseous state to diffuse through rubbermembranes. Prior art has not shown the feasibility of separatinghydrocarbons having similar molecular weights and solubilitycharacteristics. An object of this invention is to provide a process forseparating a hydrocarbon from a mixture of hydrocarbons having similarboiling points.

According to this invention it has been found that straight chainhydrocarbons in the liquid phase will diffuse through a diaphragm ofhighly impervious rubber, such as Perbunan or Neoprene, five times asfast as will the branched chain hydrocarbons even though both thehydrocarbon compounds boil at the same temperature. This eil'ect cannotbe readily obtained with a natural rubber diaphragm, since vdiiusionthrough the rubber is too rapid as compared to diffusion oral mixture ofthese two sets of compounds, so that no selectivity is obtained. It hasbeen found that by maintaining the hydrocarbons in the vapor phase,diifusion of the mixture is accelerated to such. an extent that theselectivity is preserved evenwhen the more permeable rubbers are used.This method of dlllusion has been used for the separation of normallygaseous hydrocarbons, but works equally well for normal- .2 1y liquidhydrocarbons at elevated temperatures and/or reduced pressures.

The invention will be more clearly understood upon reading the followingdescription with reference to the accompanying drawing:

A mixture of hydrocarbons, for example, a 50-50 mixture of n-heptane andiso-octane, was fed by means of tube I, through heating unit 2, intoseparating vessel l. The liquid feed was passed through a red rubbertubing I, which is the diffusing diaphragm. A pressure of 560 mm. wasmaintained inside of the rubber tubing 4, and a temperature of 212 F.was maintained outside by means of steam which was constantly passedover the exit side of the diaphragm. The nondiilusing vapor waswithdrawn rapidly by means of pipe 5, cooled in the dry ice-acetone bath6. and removed to storage. 'I'he diffused vapor was removed rapidly bymeans of pipe 1, passed through cooler 9, separator 9, and likewise,passed to storage by means of pipe I0, after the water condensate hadbeen withdrawn.

During an eight-hour run, 350 cc. of the 50-50 liquid mixture was fedinto this apparatus. The volume of dii'fusaterecovered was 76 ce. and itwas found to contain n-heptane and 25% iso-octane. The volume ofnon-diffusing liquid was 260 cc. and it was found to contain 42%n-heptane and 58% iso-octane. The separation can be enhanced to a highdegree by a multistage operation. n Y

The conditions necessary for the successful operation of this inventionare that: (1) The composition of the diffusing mixture must be keptuniform so that the concentration of the more rapidly diffusingcomponent is not depleted in that Dart of the mixture in contact withthe diaphragm, and that: (2) The concentration of the more rapidlydiffusing constituent is kept as low as possible on the opposite (exit)side of the diaphragm so that diffusion will not be reversed to anyappreciable extent. In other words, the concentration of the morerapidly diilusing constituent must be kept at a maximum on one side o1'the diaphragm and at a minimum on the other.

The uniformity of the diifusing mixture may be maintained in one of thefollowing ways: (a) The mixture maybe kept in the liquid state andsubjected to extreme agitation so that the composition of the mixture iskept constant; (b) The membrane may be made fairly impermeable so thatdiil'usion through the membrane is so slow that the composition of theliquid mixture is kept uniform by diffusion throughout the mixture; (c)The mixture iskept in the vapor phase y so that diffusion throughout themixture is facili- The membranes may be prepared either as restated,while diffusion'through the membrane is ular fllms, or by deposition ona suitable base,`

at .a slower rate. v and the selectivity may be modiiied by using com-The concentration of the more rapidly diffusing posite membranes of twoor more materials in a mixture may be kept'at a minimum on the exit 5laminated structure. l

side of the diaphragm in one of the following I claim:

vways: (a) The diffusate goes intov a receiver 1. The methodofseparating iso-octane from 6. Phenol formaldehyde. urea formaldehyde,

' 9. vulcanized natural rubber.

liquid in which the concentration of the diiusate normal heptane in aliquid mixture thereof, which is kept low by distillation or some othersuitable comprises passing the liquid mixture into contact means; (b)the diffusing mixture is heated and l with an interior wall of avulcanized rubber tube the dilusing liquid evaporates from the surfaceacting as a diaphragm maintained at a temperaof the diaphragm into inertgas or vapor, or into ture of about 212 F., and vaporizing diused apartial vacuum, in either case the diffusate being normal heptane froman outerside oi said tube, recovered by condensation. In case thediffusate and condensing the thus vaporized normal hepevaporates intoquiescent inert gas, it is advantane. tageous to have the condenser inimmediate prox- 2. The method of separating iso-octane from imity to thediaphram- If the diffusalie eVaDO- n-heptane in a liquid mixture thereofwhich comrates into a vacuum, or into a stream of inert gas. prisescontacting the mixture maintained in liquid the condenser may be locatedat any convenient phase with one side of a diaphragm of a heated pointto which the dilfusate vapor is conducted. vulcanized natural rubberthrough which n-hep- The following compositions or substances may tanediffuses at a reduced pressure, the diffused be used as diaphragms:n-heptane being vaporized and rapidly removed U posi ns Ebo from thediaphragm of vulcanized natural rubber ggllybllitlabilngom up such asnite) and the iso-octane likewise being rapidly removed, 3:Butadienetyene copolymers 25 and condensing the thus vaporized diffused4. Butadiene acrylonitrile or butadiene-methn'heptane removed from thediaphragmacrylonitrile copolymers. I ANTHONY E. ROBERTSON.

5. Polyacrylonitrile or polymethacrylonitrile. REFERENCES CITEDphenol-furfural, and similar heat-resistant The following referenlcesare of record in the resins ille of this patent:

7. Naturalrubber. UNITED NTS 8. Polymers of 2 chloro-butadiene. STATESPATE Nanne Date Number 2,159,434 Frey May 23, 1939

